1. Field of the Invention
The present invention relates to a method for brazing an element transversely to a wall and to a brazed-joint assembly for carrying out said method.
The invention is more particularly applicable to formation of a hermetically sealed and electrically insulating joint between an electric conductor and a wall of electrically conductive material through which the conductor passes.
Within this framework, the invention is primarily concerned with the construction of a package for electronic components.
2. Description of the Prior Art
Brazing of an element transversely to a wall presents a problem of supply of brazing filler metal to the surfaces to be joined together. In fact, if the brazing filler metal fails to join all the portions of surface of the transverse element to the internal surface of a receiving hole in which the element is engaged, this has an adverse effect on the mechanical strength of the assembly thus formed.
Moreover, when the objective to be achieved is provide a hermetically sealed joint connection between a wall and a so-called lead-through element which passes through the wall, discontinuities of contact of the brazing filler metal between all the oppositely-facing portions of surface are liable to result in faulty air-tightness of the joint connection.
Furthermore, in this case also, the supply of brazing filler metal is particularly complex whenever the surfaces to be brazed to each other are not directly accessible.
For this purpose, a solution which is usually adopted consists in employing an annular preform of solid brazing filler metal in which the lead-through element is engaged prior to its introduction in the receiving hole, then in causing liquefaction of the brazing metal preform while simultaneously applying a transverse pressure on the lead-through element in the direction of the wall in which the brazing metal preform is located so as to drive the liquid brazing metal into the interstice which remains between the element and the hole. This pressure is necessary in order to locate the lead-through element in its final position, taking into account the fact that the thickness of the solid brazing metal preform induces a displacement in the initial positioning of the element.
A further drawback of the solution considered above lies in the need to provide a preform of solid brazing filler metal having dimensions proportionately greater than the volume of filler metal which is necessary, this being due to the fact that, when pressure is applied, a quantity of liquid brazing filler metal is discharged from the through-hole at the end opposite to the opening through which the element is introduced. This not only affects the economic feasibility of the system but forms dirty deposits on the end product.
Furthermore, this solution does not make it possible to ensure that all the portions of surface of the element are joined by brazing to all the portions of surface of the through-hole.
It is also worthy of note that preforms of solid brazing filler metal usually contain impurities which have an adverse effect on the mechanical strength and air-tightness of the joint formed and which are introduced at the time of liquefaction within the interstice between the element and the through-hole.